In general, in an image forming apparatus, accuracy of a recording position (hereinafter referred to as “recording accuracy”) of an image with respect to a sheet is one of the important factors from the viewpoint of keeping the image quality. Therefore, for example, when a sheet being conveyed is skewed in an image formation, it is necessary to correct the skewed sheet to form an image in an appropriate position on the sheet. As described above, various sheet conveying apparatus having a skew feed correction function are proposed to improve the recording accuracy in the conventional image forming apparatus.
For example, the sheet conveying apparatus described in Patent Literature 1 includes a plurality of conveying roller pairs configured to convey a sheet to an image forming portion, which are arranged on an upstream side in a sheet conveying direction so as to be closest to the image forming portion (hereinafter simply referred to as “upstream side”), and a locking member rotatable about a rotary shaft for one roller of the conveying roller pairs. The locking member has an abutment surface against which the sheet abuts, and is urged by an urging spring so that the abutment surface is located at a home position (position at which a leading edge of a skewed sheet is brought into abutment with the abutment surface so that skew feed is corrected). When a leading edge of the conveyed sheet abuts against the abutment surface of the locking member, the sheet is locked by an urging force applied to the locking member, and the locked sheet is deflected to be curved to form a loop. The loop thus formed causes the leading edge portion of the sheet to be aligned with a width direction orthogonal to the sheet conveying direction, to thereby correct the skew feed. After that, when the locking member is pushed and rotated by stiffness of the sheet, the sheet is nipped by nips of the conveying roller pairs with the leading edge of the sheet being aligned in the width direction, and conveyed to a downstream side by the conveying roller pairs.
By the way, in recent years, much higher throughput (number of sheets subjected to image formation per unit time) of an image forming apparatus has been demanded. In order to meet this demand, it has been required to convey sheets at a higher speed, and reduce a distance between a trailing edge of a preceding sheet and a leading edge of a succeeding sheet (hereinafter referred to as “sheet-to-sheet distance”). In accordance therewith, it is necessary to return the locking member to its home position within a short sheet-to-sheet distance.
In this context, FIGS. 23A and 23B illustrate a locking member 170 provided in a conventional sheet conveying apparatus. As illustrated in FIGS. 23A and 23B, the conventional locking member 170 is supported in a manner of being reciprocally rotatable about a rotary shaft 131a of a conveying roller 131, which forms a nip N with a conveying rotatable member 132. After correcting skew feed of a sheet S by bringing a leading edge of the sheet S into abutment against an abutment surface 170a at a home position HP, the locking member 170 is rotated to guide the sheet S to the nip N. Then, after the sheet S passes through the nip N, the locking member 170 is reversely rotated to return to the home position HP.
In this case, a requisite minimum sheet-to-sheet distance is equal to a total distance of a distance D1 from a position at which a trailing edge of a preceding sheet S passes the abutment surface 170a of the locking member 170 to the home position HP at which the skew feed of the sheet S is corrected, and a distance D2 required for conveying a succeeding sheet S to the home position HP while the preceding sheet S is moved by the distance D1. The locking member 170 performs reciprocating rotation, and hence the distance D1 is generated so as to return the locking member 170 to the home position HP after the sheet S passes through the nip N, and the locking member 170 takes a time ΔT for moving the distance D1. On the other hand, the distance D2 is a distance (ΔT×V) obtained by multiplying the time ΔT during which the locking member 170 moves the distance D1 by a conveying speed V of the sheet S. As the conveying speed V of the sheet S becomes higher, the distance becomes longer. Therefore, in the conventional sheet conveying apparatus, when the conveying speed V of the sheet S is increased, the sheet-to-sheet distance needs to be set longer, and hence further enhancement of the throughput is practically impossible.
Thus, in the sheet conveying apparatus in which skew feed of sheets is corrected by using the locking member, enhancement of throughput of the sheet conveyance has been limited due to a time period for returning the locking member.